SiRNAs are synthetic double-stranded oligonucleotides of ~21 base pairs. When applied to cells, siRNAs can effectively and specifically silence their target genes, called RNA interference (RNAi). RNAi has become attractive tool for functional genomics and a potential class of molecules for human therapeutics. However, almost every siRNA drug company encounters several hurdles, mainly the high dose of siRNA needed for human trials with which come potential side effects, and off-target effects of siRNA molecules. The latter is believed to be resolvable by clever design of siRNA molecules. There are no solutions for the former at the moment. To address these difficulties associated with siRNA therapy, we have identified an RNAi-enhancer that can raise siRNA efficacy, lower the required siRNA dose, and prolong its silencing effect. If successfully developed, these enhancers could be used as an adjuvant therapy with siRNA drugs. Huntington's disease (HD) is a rare but fatal autosomal dominant neurodegenerative disorder caused by dominant mutations in the genes encoding the protein huntingtin (Htt). There are approximately 15,000 to 30,000 people in the United States who suffer from Huntington's disease. Currently there is no treatment to reverse the progression of the disease. RNAi-mediated silencing of the mutated Htt genes is a promising therapeutic treatment for HD. To date, two siRNAs have been developed for the treatment of HD. As a result, the full efficacy of current siRNA drug candidates for HD may not yet be achieved. In this Phase I STTR proposal, we will examine whether RNAi-E can enhance the efficacy of siRNAs to treat HD in a mouse model, which will provide the proof-of-principle demonstration that RNAi-E could be used as an adjuvant therapy along with siRNA drugs. We will determine the optimal concentration of an RNAi-enhancing compound (RNAi-E) to potentiate chemically modified siRNA molecules targeting Htt in an in vitro system and determine the feasibility of RNAi-E for enhancing siRNA efficacy using a mouse model. Phase II of this project will entail IND enabling formulation, pharmacokinetics and toxicology studies that advance the leading compound(s) into human clinical trials. If these studies are successful, this would be a major stride toward the improvement of siRNAs for use as therapeutic reagents. PUBLIC HEALTH RELEVANCE: This Phase I STTR application is focused on the development of an adjuvant therapy with the siRNA drugs for human diseases based on the identification of an RNAi-enhancer that can raise siRNA efficacy, lower the required siRNA dose, and prolong its silencing effect. The proposed studies would be a major stride toward the improvement of siRNAs for use as therapeutic reagents to treat human diseases. [unreadable] [unreadable] [unreadable]